WHAT: Defining a population is multi-faceted


  • A group of organisms from one species…
    • abundance
    • sex ratios
    • age structure
    • density


  • Defining the population boundary…
    • a lake
    • ecosystem
    • country or state


  • Aphid population: leaf, plant, grassland, ecosystem?
    • depends on the study and the questions

WHY: Understanding population structure (Demography)


  • Is population size ↑ ←→ ↓ ?


  • Age of a population, peak reproductive ages


  • Frequency of reproduction, reproductive output


  • Sex ratios determining mating potential


  • Measuring, predicting and evaluating population structure/sizes guides decisions
    • conservation, pest management, harvesting

Case Study: Deer population size

Case Study: Deer population density matters too…

Hunting regulations, disease vectors, car insurance…

Case Study: Zombie deer and population density


WHO: Definition of an individual can be tricky



  • Unitary organism: forming a single entity
    • deer, bird, spider, etc.


  • Modular organisms: colonies
    • bacteria, algae, corals, biofilms
    • plants: ferns grow by rhizomes


  • Some modular organisms may themselves be populations

HOW: Estimates from representative samples




  • Most methods are indirect estimates



  • Lessons from Lab:
    • Census techniques
    • Mark & Recapture
    • Spatial component
    • Temporal component

Populations dynamics: Movement (dispersal and migration)


Life history traits are super variable across species

Reproductive timing differs across organisms

Semelparous: One-off reproduction

Making offspring is expensive, so reproductive output is a strategy under natural selection

Species with short life cycles: Annuals & Ephemerals

Making offspring is expensive, so reproductive output is a strategy under natural selection

Populations and life cycles: Dormancy


  • Annual organism may spend part of the year dormant
    • Seeds, Spores, Cysts, etc.


  • Thousands of seeds per square meter of soil


  • Dormancy can last for long periods of time
    • protection from harsh environments


  • How does this affect population estimates?
    • Reproductive effort and offspring may not be synced

Life History: Reproductive conflict


  • Goal to pass on genes…









  • When and how to spend your resources
    • reproduction comes at a cost of growth
    • ↑ flowers + seeds = ↓ leaves and roots
    • are enough resources available to support pregnancy in animals?
    • surviving vs growing the population…
  • Life histories of males and females may differ due to reproductive investment















  • Populations are composites of individual life histories
    • who survived from previous year
    • new individuals (births/immigration)
    • survivability of offspring and parents in a year


Survivorship =

Populations dynamics: Monitoring birth and death



  • Measuring rates of birth rates and survivorship tell us if a population is growing or shrinking


  • Cohort Life tables: follow the fate of individuals from a single cohort
    • survivorship of every individual
    • practically difficult (e.g., mobile animals)


  • Static life table: count # of survivors at different ages


  • Tells us a lot about age structure of a population

r/K selection theory: Growth and reproduction strategies





  • r-selected species: “cheap” offspring


  • K-selected species: “expensive” offspring



  • These are the 2 ends of a spectrum

Populations with unlimited resources: Exponential growth

Population growth eventually faces resistance


  • Density-dependent
    • competition
    • predation
    • disease
    • resources


  • Density-independent
    • weather events
    • disturbances


  • Carrying Capacity:
    • environmental resistance leads to Logistic growth

Population cycles are often tied to other species